Catadioptric projection objective with mirror group

ABSTRACT

A catadioptric projection objective for imaging an off-axis object field arranged in an object surface of the projection objective onto an off-axis image field arranged in an image surface of the projection objective has a front lens group, a mirror group comprising four mirrors and having an object side mirror group entry, an image side mirror group exit, and a mirror group plane aligned transversely to the optical axis and arranged geometrically between the mirror group entry and the mirror group exit; and a rear lens group.

This application claims priority from U.S. provisional application60/560,267 filed on Apr. 8, 2004. The complete disclosure of thatprovisional application is incorporated into this application byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a catadioptric projection objective for imagingan off-axis object field arranged in an object surface of the projectionobjective onto an off-axis image field arranged in an image surface ofthe projection objective while creating at least one intermediate image.

2. Description of the Related Art

Catadioptric projection objectives are, for example, employed inprojection exposure systems, in particular wafer scanners or wafersteppers, used for fabricating semiconductor devices and other types ofmicro-devices and serve to project patterns on photomasks or reticles,hereinafter referred to generically as “masks” or “reticles,” onto anobject having a photosensitive coating with ultrahigh resolution on areduced scale.

In order to create even finer structures, it is sought to both increasethe image-end numerical aperture (NA) of the projection objective andemploy shorter wavelengths, preferably ultraviolet light withwavelengths less than about 260 nm. However, there are very fewmaterials, in particular, synthetic quartz glass and crystallinefluorides, that are sufficiently transparent in that wavelength regionavailable for fabricating the optical elements. Since the Abbe numbersof those materials that are available lie rather close to one another,it is difficult to provide purely refractive systems that aresufficiently well color-corrected (corrected for chromatic aberrations).

The high prices of the materials involved and limited availability ofcrystalline calcium fluoride in sizes large enough for fabricating largelenses represent problems. Measures that allow reducing the number andsizes of lenses employed and simultaneously contribute to maintaining,or even improving, imaging fidelity are thus desired.

In optical lithography, high resolution and good correction status haveto be obtained for a relatively large, virtually planar image field. Ithas been pointed out that the most difficult requirement that one canask of any optical design is that it have a flat image, especially if itis an all-refractive design. Providing a flat image requires opposinglens powers and that leads to stronger lenses, more system length,larger system glass mass, and larger higher-order image aberrations thatresult from the stronger lens curvatures. Conventional means forflattening the image field, i.e. for correctings the Petzval sum inprojection objectives for microlithography are discussed in the article“New lenses for microlithography” by E. Glatzel, SPIE Vol. 237 (1980),pp. 310-320.

Concave mirrors have been used for some time to help solve problems ofcolor correction and image flattening. A concave mirror has positivepower, like a positive lens, but the opposite sign of Petzval curvature.Also, concave mirrors do not introduce color problems.

Therefore, catadioptric systems that combine refracting and reflectingelements, particularly lenses and concave mirrors, are primarilyemployed for configuring high-resolution projection objectives of theaforementioned type.

Unfortunately, a concave mirror is difficult to integrate into anoptical design, since it sends the radiation right back in the directionit came from. Intelligent designs integrating concave mirrors withoutcausing mechanical problems or problems due to beam vignetting or pupilobscuration are desirable.

When using one or more concave mirrors for correcting the Petzval sum ofan imaging system it is desirable that the contribution of the concavemirror to Petzval sum correction is just sufficient to compensateopposing effects of other parts of the projection objective. Thecontribution to Petzval sum should not be too weak or too strong.Therefore, optical design concepts allowing for a certain amount offlexibility of Petzval sum correction are desirable.

One type of catadioptric group frequently used in projection objectivesfor microlithography is a combination of a concave mirror arranged closeto or at a pupil surface and one or more negative lenses arranged aheadof the concave mirror and passed twice by radiation. The Petzval sum ofthis type of catadioptric group can be varied by changing the refractivepower of the lenses and the concave mirror while maintaining anessentially constant refractive power of the entire catadioptric group.This is one fundamental feature of the Schupmann-Achromat, which isutilized in some types of catadioptric projection objectives, forexample those using geometrical beam splitting with one or more planarfolding mirrors for guiding radiation towards the catadioptric groupand/or for deflecting radiation emanating from the catadioptric group.Representative examples for folded catadioptric projection objectivesusing planar folding mirrors in combination with a single catadioptricgroup as described above are given in US 2003/0234912 A1 or US2004/0160677 A1.

A number of catadioptric projection objectives having one straight(unfolded) optical axis common to all optical elements of the projectionobjective have been proposed, which will be denoted as “in-line systems”in the following. From an optical point of view, in-line systems may befavorable since optical problems caused by utilizing planar foldingmirrors, such as polarization effects, can be avoided. Also, from amanufacturing point of view, in-line systems may be designed such thatconventional mounting techniques for optical elements can be utilized,thereby improving mechanical stability of the projection objectives.

The patent U.S. Pat. No. 6,600,608 B1 discloses a catadioptric in-lineprojection objective having a first, purely refractive objective partfor imaging a pattern arranged in the object plane of the projectionobjective into a first intermediate image, a second objective part forimaging the first intermediate image into a second intermediate imageand a third objective part for imaging the second intermediate imagedirectly, that is without a further intermediate image, onto the imageplane. The second objective part is a catadioptric objective part havinga first concave mirror with a central bore and a second concave mirrorwith a central bore, the concave mirrors having the mirror faces facingeach other and defining an intermirror space or catadioptric cavity inbetween. The first intermediate image is formed within the central boreof the concave mirror next to the object plane, whereas the secondintermediate image is formed within the central bore of the concavemirror next to the object plane. The objective has axial symmetry and afield centered around the optical axis and provides good colorcorrection axially and laterally. However, since the reflecting areas ofthe concave mirrors exposed to the radiation are interrupted at thebores, the pupil of the system is obscured.

The Patent EP 1 069 448 B1 discloses catadioptric projection objectiveshaving two concave mirrors facing each other and an off-axis object andimage field. The concave mirrors are part of a first catadioptricobjective part imaging the object onto an intermediate image positionedadjacent to a concave mirror. This is the only intermediate image, whichis imaged to the image plane by a second, purely refractive objectivepart. The object as well as the image of the catadioptric imaging systemare positioned outside the intermirror space defined by the mirrorsfacing each other. Similar systems having two concave mirrors, a commonstraight optical axis and one intermediate image formed by acatadioptric imaging system and positioned besides one of the concavemirrors is disclosed in US patent application 2002/0024741 A1.

US patent application 2004/0130806 (corresponding to European patentapplication EP 1 336 887) diicloses catadioptric projection objectiveshaving off-axis object and image field, one common straight optical axisand, in that sequence, a first catadioptric objective part for creatinga first intermediate image, a second catadioptric objective part forcreating a second intermediate image from the first intermediate image,and a refractive third objective part forming the image from the secondintermediate image. Each catadioptric system has two concave mirrorsfacing each other. The intermediate images lie outside the intermirrorspaces defined by the concave mirrors.

Japanese patent application JP 2003114387 A and international patentapplication WO 01/55767 A disclose catadioptric projection objectiveswith off-axis object and image field having one common straight opticalaxis, a first catadioptric objective part for forming an intermediateimage and a second catadioptric objective part for imaging theintermediate image onto the image plane of this system. Concave andconvex mirrors are used in combination.

US patent application 2003/0234992 A1 discloses catadioptric projectionobjectives with off-axis object and image field having one commonstraight optical axis, a first catadioptric objective part for formingan intermediate image, and a second catadioptric objective part forimaging the intermediate image onto the image plane. In eachcatadioptric objective part concave and convex mirrors are used incombination with one single lens.

International patent application WO 2004/107011 A1 discloses variouscatadioptric projection objectives with off-axis object field and imagefield having one common straight optical axis designed for immersionlithography using an arc shaped effective object field. The projectionobjecLives include various types of mirror groups having two, four orsix curved mirrors. Embodiments with one or two intermediate images aredisclosed.

U.S. provisional application with Ser. No. 60/536,248 filed on Jan. 14,2004 by the applicant discloses a catadioptric projection objectivehaving very high NA and suitable for immersion lithography at NA>1. Theprojection objective comprises: a first objective part for imaging thepattern provided in the object plane into a first intermediate image, asecond objective part for imaging the first intermediate imaging into asecond intermediate image, and a third objective part for imaging thesecond intermediate imaging directly onto the image plane. The secondobjective part includes a first concave mirror having a first continuousmirror surface and a second concave mirror having a second continuousmirror surface, the concave mirror surfaces facing each other anddefining an intermirror space. All mirrors are positioned opticallyremote from a pupil surface. The system has potential for very highnumerical apertures at moderate lens mass consumption. A limitedflexibility for Petzval sum correction provided by the concave mirrorsis given since vignetting problems have to be observed when therefractive power of the concave mirrors is adjusted.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a catadioptric in-lineprojection objective allowing flexibility for Petzval sum correction atmoderate variations of the overall design of the projection objective.It is another object of the invention to provide a catadioptric in-lineprojection objective that can be built with relatively small amounts oftransparent optical material. It is another object of the invention toprovide a catadioptric in-line projection objective for microlithographysuitable for use in the vacuum ultraviolet (VUV) range having potentialfor very high image side numerical aperture which may extend to valuesallowing immersion lithography at numerical apertures NA>1. It isanother object of the invention to provide a catadioptric in-lineprojection objective having an axially compact arrangement of mirrorseffective for compensating image curvature abberations caused by lenseswith positive power within the projection objective.

As a solution to these and other objects the invention, according to oneformulation, provides a catadioptric projection objective for imaging anoff-axis object field arranged in an object surface of the projectionobjective onto an off-axis image field arranged in an image surface ofthe projection objective while creating at least one intermediate imagecomprising in that order along an optical axis:

a front lens group having positive refractive power for convergingradiation coming from the object field towards a mirror group entry of amirror group;the mirror group having the object side mirror group entry, an imageside mirror group exit, and a mirror group plane defined transversly tothe optical axis and arranged geometrically between the mirror groupentry and the mirror group exit; anda rear lens group with positive refractive power for focusing radiationemerging from the mirror group exit onto the image surface;the mirror group having:a first mirror for receiving radiation from the mirror group entry on afirst reflecting area;a second mirror for receiving radiation reflected from the first mirroron a second reflecting area;a third mirror for receiving radiation reflected from the second mirroron a third reflecting area;and a fourth mirror for receiving radiation reflected from the thirdmirror on a fourth reflecting area and for reflecting the radiation tothe mirror group exit;at least two of the mirrors being concave mirrors having a concavemirror surface having a surface of curvature rotationally symmetric tothe optical axis; wherein:the mirrors of the mirror group are arranged such that at least oneintermediate image is positioned inside the mirror group between mirrorgroup entry and mirror group exit, and that radiation coming from themirror group entry passes at least four times through the mirror groupplane and is reflected at least twice at a concave mirror surface of themirror group prior to exiting the mirror group at the mirror group exit;the mirror group entry is positioned in a region where radiation exitingthe front lens group has an entry chief ray height;the second reflecting area is positioned in a region where radiationimpinging on the second mirror has a second chief ray height deviatingfrom the entry chief ray height in a first direction; andthe fourth reflecting area is positioned in a region where radiationimpinging on the fourth mirror has a fourth chief ray height deviatingfrom the entry chief ray height in a second direction opposite to thefirst direction.

In this formulation, the term “direction” refers to directions along aray of numbers in a sense that the first direction may be the directionof increasing numbers whereas the second direction is the direction ofdecreasing numbers or vice versa. If, for example, the entry chief rayheight equals zero (i.e. chief ray on the optical axis at the mirrorgroup entry), then the second chief ray height may be positive and thefourth chief ray height may be negative or the other way round. If theentry chief ray height has a finite positive value, then one of thesecond and fourth chief ray height will have a larger positive value andthe other, chief ray height may have a smaller positive value or anegative value, or may be zero. Analogously, if the entry chief rayheight has a finite negative value, then one of the second and fourthchief ray height will have a more negative value and the other,remaining chief ray height may have a less negative value or a positivevalue or may be zero.

In a design where the chief ray positions of the second and fourthreflection (both on object-side mirrors having mirror surfaces facingthe image side) occur at opposite sides relative to the position of thechief ray at the mirror group entry, a design space can be used allowingoptical designs optimized with regard to effective use of concavemirrors.

Since the radiation entering the mirror group is reflected at leasttwice on a concave mirror surface before exiting the mirror group, themirror group can provide strong overcorrection of the Petzval sum, whichcan at least partly compensate opposite effects on, image curvaturecaused by positive refractive power upstream and/or downstream of themirror group. The mirror group can, therefore, be regarded as a “Petzvalsum corrector”. The mirror group can be modified with regard tocurvature and relative position of the mirrors in order to modify theamount of Petzval sum provided by the mirror group with only a limitedeffect on the course of the projection beam within the remainder partsof the projection objective, whereby the design can be optimized suchthat a suitable distribution of means for correcting Petzval sum withinthe system can be chosen as needed.

Since the mirrors of the mirror group are arranged such that radiationcoming from the mirror group entry passes at least four times throughthe mirror group plane prior to exiting the mirror group at the mirrorgroup exit, a multitude of at least four reflections can be obtainedwithin an axially compact space defined between the mirror group entryand the mirror group exit. The mirror group plane may be a planeperpendicular to the optical axis and positioned between the vertices ofthe first and the second mirror of the mirror group.

In some embodiments, exactly four reflections occur within the mirrorgroup providing a good compromise between a desired influence ofreflections of the field curvature and an undesired loss of intensityinvolved with each reflection on a mirror.

In some embodiments of this type the first, second, third and fourthmirror is a concave mirror, thus providing four reflections on concavemirror surfaces. Strong Petzval overcorrection can be obtained this waysince each reflection contributes a certain amount of Petzvalovercorrection.

In some embodiments, a length ratio LR between an axial mirror grouplength MGL and a total track length TT of the projection objective isless than 50%, where the mirror group length is the axial distancebetween a mirror vertex closest to the object surface and a mirrorvertex closest to the image surface and the total track length is theaxial distance between object surface and the image surface. PreferablyLR=MGL/TT is less than 40% or less than 30%, indicating axially compactmirror groups allowing to integrate strong Petzval correction in opticaldesigns with moderate track length.

There are different possibilities to integrate a mirror group into theprojection objective.

In some embodiments the mirror group entry includes the optical axis andthe positions of the chief rays of the second and fourth reflection arepositioned on opposite sides of the optical axis.

In some embodiments it has been found beneficial to integrate the mirrorgroup into the overall design such that the mirror group entry ispositioned geometrically close to a front pupil surface of theprojection objective. In this case, the projection beam (i.e. theradiation beam emanating from the object field and running to the imagefield) includes the optical axis in the region of the mirror groupentry. An axial position of the mirror group entry “in the vicinity of apupil surface” may particularly be defined as an axial position wherethe chief ray height CRH is smaller than the marginal ray height MRH.

The marginal ray is a ray running from an axial field point (on theoptical axis) to the edge of an aperture stop. In an off-axis system themarginal ray may not contribute to the formation of an image due tovignetting. The chief ray (also known as principal ray) is a ray runningfrom an outermost field point (farthest away from the optical axis) andintersecting the optical axis at a pupil surface position. Due torotational symmetry of a projection objective the outermost field pointmay be chosen from an equivalent field point on the meridional plane.

The front lens group arranged between the object surface and the mirrorgroup entry allows to transform the spatial distribution of radiation atthe object surface into a desired angular distribution of radiation atthe mirror group entry and to adjust the angles of incidence with whichthe radiation enters the mirror group and impinges on the first mirror.Also, the design of the front lens group is selected such that theradiation beam entering the mirror group entry has a desiredcross-sectional shape allowing to pass the radiation beam into themirror group exit without hitting adjacent mirror edges, therebyavoiding vignetting of the beam.

In embodiments having the mirror group entry including the optical axis,particularly where the mirror group entry lies geometrically close to apupil surface, the front lens group may be designed as a Fourier lensgroup. The term “Fourier lens group” as used here refers to a singleoptical element or to a group consisting of at least two opticalelements which perform one single Fourier transformation or an oddnumber of consecutive Fourier transformations between a front focalplane and a rear focal plane of the Fourier lens group. A Fourier lensgroup may be all refractive consisting of one or more transparentlenses. A Fourier lens group may also be purely reflective consisting ofone or more mirrors, at least some of the mirrors being curved mirrors.Catadioptric Fourier lens groups combining transparent lenses andmirrors are also possible. In preferred embodiments a Fourier lens groupforming the front lens group is purely refractive and performs a singleFourier transformation.

The front lens group may be axially compact having an axial length whichmay be less than 40% or less than 30% or less than 25% of the totaltrack length of the projection objective.

There are different possibilities to place the mirror group exit.According to one embodiment the mirror group exit is arrangedgeometrically close to a rear pupil surface optically conjugate to thefront pupil surface. In this case, the mirror group is designed toperform a pupil imaging between mirror group entry and mirror groupexit. At least one intermediate image is thereby formed within themirror group. Preferably, more than one intermediate image, e.g. two orthree intermediate images, are formed within the mirror group. A mirrorgroup exit close to a pupil surface allows to place the exit such thatthe optical axis is included into the projection beam at the mirrorgroup exit, thereby allowing moderate lens diameters downstream of themirror group exit.

The terms “front” and “rear” relate to the position along the opticalpath, wherein a front pupil surface lies upstream of a rear pupilsurface. Geometrically, a front pupil surface will normally be closer tothe object surface, whereas a rear pupil surface will normally be closerto the image surface.

Where the mirror group exit is positioned geometrically close to a pupilsurface, a Fourier lens group for forming an intermediate image in aconstriction region may be provided immedeately downstream of the mirrorgroup exit. In this case, a subsequent lens group may be designed as animaging subsystem for imaging the intermediate image formed by theFourier lens group onto the image surface on a reduced scale.

In other embodiments, the mirror group exit is arranged outside theoptical axis optically close to an intermediate image (i.e. opticallyremote from a pupil surface), and the second and third lens groupcombined form an imaging subsystem for imaging that intermediate imageonto the image surface on a reduced scale. This type of embodimentsgenerally allows for smaller track length, however, larger lenses arerequired immediately downstream of the mirror group exit for convergingthe off-axis projection beam towards the constriction region.

Preferably, at least two negative lenses are arranged in theconstriction region in embodiments having a mirror group exit opticallyremote from the pupil surface. Thereby, a predefined amount of Petzvalsum correction is contributed by small negative lenses within a purelyrefractive section of the projection objective. Since a limited amountof Petzval sum correction can thereby by provided within the refractivesection downstream of the mirror group, concave mirrors having moderatecurvatures can be utilized within the mirror group.

According to one embodiment the mirror group includes at least onemirror pair consisting of two concave mirrors having mirror surfacessharing a common surface of curvature provided on a common mirrorsubstrate having a transmissive portion or aperture provided between theconcave mirrors of the mirror pair. The concave mirrors of a mirror pairmay also be described as a concave mirror having a transmissive aperturewherein non-overlapping reflecting areas of the mirror on both sides ofthe aperture are used. The transmissive aperture is effective to allowradiation to enter or exit an intermirror space axially defined by themirror pair on one side and one or more mirrors on the other side. Amirror pair formed by concave mirrors having a common surface ofcurvature may facilitate manufacturing and mounting of the concavemirrors. In embodiments having a mirror group entry and/or a mirrorgroup exit near a pupil surface, the aperture of the mirror pairincludes the optical axis.

In some embodiments, the second and fourth mirror (i.e. the mirrorsgeometrically closest to the object surface) form a mirror pair providedon a common mirror substrate. An opening or aperture in the mirrorsubstrate may include the optical axis and may define the mirror groupentry. Likewise, it is possible that the first and third mirrors form amirror pair provided on a common mirror substrate. An opening (aperture)provided between the first and third mirror may include the optical axisand may form the mirror group exit. In some embodiments, both theobject-side mirrors (second and fourth mirror) as well as the image-sidemirrors (first and third mirror) each form a mirror pair on a commonmirror substrate such that only two mirror substrates are necessary toprovide four mirrors. Manufacturing and mounting is greatly facilitatedthis way.

The previous and other properties can be seen not only in the claims butalso in the description and the drawings, wherein individualcharacteristics may be used either alone or in sub-combinations as anembodiment of the invention and in other areas and may individuallyrepresent advantageous and patentable embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a lens section of an embodiment of a catadioptric immersionobjective for microlithography comprising a four-mirror-mirror group(type H in FIG. 8) and four intermediate images;

FIG. 2( a) to (b) shows the projection objective of FIG. 1 withdifferent representations of the concave mirrors of the mirror group andthe projection beam;

FIG. 3 shows a variant of the projection objective of FIG. 1 havingclosely spaced inner mirrors of the mirror group;

FIG. 4 shows a variant of the projection objective of FIG. 1 where twoobject-side concave mirrors are formed on a common substrate;

FIG. 5 shows a variant of the projection objective of FIG. 1 where twoimage-side concave mirrors of the mirror group are formed on one commonmirror substrate;

FIG. 6 shows a variant of the projection objective of FIG. 1 where theobject-side concave mirrors and the image-side concave mirrors each haveidentical vertex positions and different curvatures;

FIG. 7 shows a variant of the projection objective in FIG. 1 having anasymmetric arrangement of concave mirrors of the mirror group;

FIG. 8( a) to (d) shows a schematic representation of a design space formirror groups suitable for incorporation into a catadioptric projectionobjective, where different variants are distinguished by different pathsof the projection beam between mirror group entry and mirror group exit;

FIG. 9 shows a mirror group with parabolic concave mirrors and a beampath of a low aperture beam (a) and a high aperture beam (b);

FIG. 10 shows and embodiment of a catadioptric projection objective oftype F in FIG. 8 having three intermediate images;

FIG. 11 shows a variant of the projection objective of FIG. 10 where themirrors of the mirror group are disposed symmetrically about a mirrorgroup plane;

FIG. 12 shows a variant of the projection objective of FIG. 11 or 12where the first and fourth mirrors are planar mirrors perpendicular tothe optical axis;

FIG. 13 shows a variant of the projection objective of FIG. 12 with theplanar mirrors removed to form a two-mirror mirror-group;

FIG. 14 shows a representation of the projection objective of FIG. 13where effectively used areas of the first and second mirror forming themirror group are shown;

FIG. 15 shows representations of a four-mirror-mirror-group having anobject-side mirror group entry around the optical axis and an off-axismirror group exit and a path of a low aperture beam (a) and a highaperture beam (b) through the mirror group;

FIG. 16 shows an embodiment of a catadioptric projection objective withtwo intermediate images where the mirror group of FIG. 15 isincorporated;

FIG. 17 shows a variant of the projection objective of FIG. 16 with asmaller curvature of the first and third mirrors and a convergent beamat the mirror group exit;

FIG. 18 shows a variant of the projection objective of FIG. 17 havingalmost planar first and third mirrors and a convergent beam at theoff-axis mirror group exit;

FIG. 19 shows a schematic representation of a catadioptric projectionobjective suitable for immersion lithography having a mirror group withfour concave mirrors arranged in two subsequent pairs of two concavemirrors, the mirrors of a pair being disposed on different sides of theoptical axis;

FIG. 20 shows various optional variants of the embodiment of FIG. 19where one or more lenses are inserted into the mirror group;

FIG. 21 shows a variant of the embodiments of FIG. 19 or 20 having afront lens group providing a front pupil surface outside the front lensgroup;

FIG. 22 shows an embodiment of a catadioptric projection objectivesuitable for immersion lithography having one single intermediate imagewithin a mirror group formed by two concave mirrors and two convexmirrors;

FIG. 23 shows a schematic representation of a double-channelcatadioptric projection objective having two optical channels onoppolite sides of the optical axis provided by a catoptric mirror grouphaving concave and convex mirrors; and

FIG. 24 shows an axial view of the object surface of a double-channelprojection objective having rectangular object fields (a) or arcuateobject fields (b).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of preferred embodiments of the invention,the term “optical axis” shall refer to a straight line or sequence ofstraight-line segments passing through the centers of curvature of theoptical elements involved. The optical axis is folded by reflectivesurfaces. In the case of those examples presented here, the objectinvolved is either a mask (reticle) bearing the pattern of an integratedcircuit or some other pattern, for example, a grating pattern. In theexamples presented here, the image of the object is projected onto awafer serving as a substrate that is coated with a layer of photoresist,although other types of substrate, such as components of liquid-crystaldisplays or substrates for optical gratings, are also feasible.

Embodiments having a plurality of mirrors are described. Unless statedotherwise, the mirrors will be numbered according to the sequence inwhich the radiation is reflected on the mirrors. With other words, thenumbering of the mirrors denotes the mirrors according to the positionalong the optical path of radiation, rather than according togeometrical position.

Where appropriate, identical or similar features or feature groups indifferent embodiments are denoted by similar reference identifications.Where reference numerals are used, those are increased by 100 ormultiples of 100 between embodiments.

Where tables are provided to disclose the specification of a designshown in a figure, the table or tables are designated by the samenumbers as the respective figures.

In all embodiments given below the surfaces of curvature of all curvedmirrors have a common axis of rotational symmetry, also denoted mirrorgroup axis. The mirror group axis coincides with the optical axis OA ofthe imaging system. Axially symmetric optical systems, also namedcoaxial systems or in-line systems, are provided this way. Objectsurface and image surface are parallel. An even number of reflectionsoccurs. The effectively used object field and image field are off-axis,i.e. positioned entirely outside the optical axis. All systems have acircular pupil centered around the optical axis thus allowing use asprojection objectives for microlithography.

FIG. 1 shows a lens section of an embodiment of a catadioptricprojection objective 100 designed to project an image of a pattern on areticle arranged in the planar object surface OS (object plane) onto aplanar image surface IS (image plane) on a reduced scale, for example4:1, while creating exactly four real intermediate images IMI1, IMI2,IMI3 and IMI4. An off-axis object field OF positioned outside theoptical axis OA is thereby projected on an off-axis image field IF. Thepath of the chief ray CR of an outer field point of the off-axis objectfield OF is drawn bold in FIG. 1 in order to facilitate following thebeam path.

For the purpose of this application, the term “chief ray” (also known asprincipal ray) denotes a ray emanating from an outermost field point(farthest away from the optical axis) of the effectively used objectfield OF and intersecting the optical axis at at least one pupil surfaceposition. Due to the rotational symmetry of the system the chief ray maybe chosen from an equivalent field point in the meridional plane asshown in the figures for demonstration purposes. In projectionobjectives being essentially telecentric on the object side, the chiefray emanates from the object surface parallel or at a very small anglewith respect to the optical axis. The imaging process is furthercharacterized by the trajectory of marginal rays. A “marginal ray” asused herein is a ray running from an axial object field point (on theoptical axis) to the edge of an aperture stop AS. That marginal ray maynot contribute to image formation due to vignetting when an off-axiseffective object field is used. The chief ray and marginal ray arechosen to characterize optical properties of the projection objectives.

FIG. 2( a) shows a different representation of the projection objective100 with the surfaces of curvature of the concave mirrors extendedacross the optical axis to facilitate understanding of the arrangementand design of the concave mirrors. FIG. 2( b) shows yet anotherrepresentation with a beam bundle emanating from an outermost objectfield point in order to facilitate understanding the positions of theintermediate images and some properties of the projection beam passingthrough the projection objective.

A first lens group LG1 immediately following the object surface havingpositive refractive power provided by five lenses acts as an imagingsubsystem to form the first intermediate image IMI1. A front pupilsurface FPS formed between object surface and first intermediate imageis positioned outside and downstream of the first lens group LG1 at anaxial position where the chief ray CR intersects the optical axis OA. Anaperture stop may be arranged at the front pupil surface, if desired.

A purely reflective (catoptric) mirror group MG consisting of fourseparate concave mirrors M1, M2, M3 and M4 arranged mirror symmetricallywith respect to a mirror group plane MGP perpendicular to the opticalaxis is designed to form a second intermediate image IMI2 from the firstintermediate image, and a third intermediate image IMI3 from the secondintermediate image. All intermediate images IMI1, IMI2, IMI3 arepositioned inside a cavity defined by the surfaces of curvature of theconcave mirrors.

A second lens group LG2 having positive refractive power provided by sixlenses is an imaging subsystem forming a fourth intermediate image IMI4from the third intermediate image IMI3. A pupil surface RPS formedbetween the third and fourth intermediate image lies outside the lensesof the second lens group immediately upstream of the entry surface ofthe first lens of that group.

A third lens group LG3 having positive refractive power provided byeleven lenses (only two weak negative lenses) is designed as a focusinglens group with reducing magnification to image the fourth intermediateimage IMI4 onto the image surface IS on a reduced scale.

A constriction region CON characterized by a local minimum of beamdiameter is defined between the second and third lens group LG2 and LG3including the position of the fourth intermediate image IMI4.

The first lens group LG1 forms a front lens group FLG designed toconverge the radiation coming from the object field towards the mirrorgroup entry. The second lens group LG2 and the third lens group LG3 incombination serve as a rear lens group RLG for focusing the radiationemerging from the mirror group exit MGO onto the image surface.

The purely reflective (catoptric) mirror group MG is designed to providestrong overcorrection of the Petzval sum counteracting opposite effectsof positive refractive power of lenses upstream and downstream of themirror group. To that end, the mirror group MG consists of a firstconcave mirror M1 placed on the side of the optical axis opposite to theobject field OF, a second concave mirror M2 placed on the object fieldside of the optical axis, a third concave mirror M3 also placed on theobject field side of the optical axis, and a fourth concave mirror M4placed on the side opposite to the object field. A finite axial distance(vertex distance) exists between the intersections of the surfaces ofcurvature of the most object side mirror (M4) and the geometricallyclosest mirror (M2) on the opposite side of the optical axis. A mirrorgroup entry MGI is formed between the mutually facing edges of mirrorsM2 and M4. As the mirror arrangement is mirror symmetric to a symmetryplane (mirror group plane MOP) perpendicular to the optical axis,symmetric conditions are given on the exit side, where a mirror groupexit MOO is formed between the third mirror M3 closer to the object andthe first mirror M1 closer to the image-side. Both mirror group entryMGI and mirror group exit MGO include the optical axis.

The mirror group entry MGI has an axial position geometrically close tothe front pupil surface FPS. Since the chief ray height (i.e. the radialdistance between the optical axis and the chief ray) equals zero at thefront pupil surface, an entry chief ray height CRHI at the mirror groupentry is small in absolute values. In FIG. 1, a dot T illustrates theposition where the chief ray transits the mirror group entry. Thiscorresponds to a small negative value. After reflection on the firstmirror M1, the radiation beam crosses the optical axis and is incidenton the second mirror M2. The second reflecting area (footprint) on thesecond mirror includes the position R2 (dot) where the chief rayimpinges on the second mirror. The corresponding second chief ray heightCRH2 is larger than the first chief ray height (the ray height beingdetermined in a radial direction to the optical axis with positivevalues on the side of the object field in this case). Specifically, CRH2has a positive value. After forming the second intermediate image andreflection on the third mirror M3, the radiation beam crosses theoptical axis again and is incident on the fourth mirror M4 in a fourthreflecting area including the position R4 (dot) where the chief ray isreflected on the fourth mirror. The corresponding fourth chief rayheight CRH4 is smaller than the entry chief ray height CRHI since it hasa more negative value than CRHI. Also, the second and fourth chief rayheights CRH2 and CRH4 have opposite signs, CRH2 being positive and CRH4being negative.

The small absolute value of entry chief ray height CRHI indicates closevicinity of the mirror group entry to a pupil surface. In contrast, highabsolute values of the chief ray heights for the second and fourthreflection indicate that these reflections occur optically remote from apupil surface optically closer to a field surface nearby (IMI2 for thesecond reflection, and IMI3 for the fourth reflection). Due to thesymmetry of the mirror group, the reflections on the first and thirdmirrors M1, M3 are also closer to field surfaces than to a pupil surfaceindicating that all reflections within the mirror group occur close to afield surface optically remote from a pupil surface.

Due to the symmetry of the mirror group, the front pupil surface FPS ispositioned near the mirror group entry, whereas the optically conjugaterear pupil surface RPS lies near the mirror group exit. Inside themirror group, three intermediate images (corresponding to fieldsurfaces) are positioned. When viewed along the light propagation path,the first intermediate image IMI1 is positioned upstream of the firstreflection at M1, the second intermediate image IMI2 is positionedbetween the second and the third reflection between mirrors M2 and M3,and a third intermediate image IMI3 is positioned immediately downstreamof the fourth reflection at M4. The mirror group plane MOP is passedfive times by the projection beam between mirror group entry and mirrorgroup exit.

An axial mirror group length MGL defined as the axial distance between amirror vertex closest to the object surface (mirror M4) and a mirrorvertex closest to the image surface (mirror M1) is less than 30% of thetotal track length TT of projection objective (axial distance betweenobject surface and image surface), indicating an axially compact mirrorgroup.

The second intermediate image is essentially telecentric indicated bythe fact that the chief ray CR runs almost parallel to the optical axisin the region of the second intermediate image. An essentiallycollimated beam is formed between the first and second mirrors M1, M2,forming a second pupil surface P2 close to the focal point of the secondmirror. Likewise a collimated beam is present between the third andfourth mirror M3, M4, forming a third pupil surface P3 near the focalpoint of the third mirror M3.

The projection objective 100 is designed as an immersion objective forλ=193 nm having an image side numerical aperture NA=1.20 when used inconjunction with a high index immersion fluid, e.g. pure water, betweenthe exit face of the objective and the image plane. The size of therectangular field is 26 mm*5.5 mm. Specifications are summarized inTable 1. The leftmost column lists the number of the refractive,reflective, or otherwise designated surface, the second column lists theradius, r, of that surface [mm], the third column lists the distance, d[mm], between that surface and the next surface, a parameter that isreferred to as the “thickness” of the optical element, the fourth columnlists the material employed for fabricating that optical element, andthe fifth column lists the refractive index of that material. The sixthcolumn lists the optically utilizable, clear, semi diameter [mm] of theoptical component. A radius r=0 in a table designates a planar surface(having infinite radius).

A number of surfaces in table 1, are aspherical surfaces. Table 1A liststhe associated data for those aspherical surfaces, from which thesagitta or rising height p(h) of their surface figures as a function ofthe height h may be computed employing the following equation:

p(h)=[((1/r)h ²)/(1+SQRT(1−(1+K)(1/r)² h ²))]+C1·h ⁴ +C2·h ⁶+,

where the reciprocal value (1/r) of the radius is the curvature of thesurface in question at the surface vertex and h is the distance of apoint thereon from the optical axis. The sagitta or rising height p(h)thus represents the distance of that point from the vertex of thesurface in question, measured along the z-direction, i.e., along theoptical axis. The constants K, C1, C2, etc., are listed in Table 1A.

Some considerations for obtaining a high geometrical light conductancevalue (etendue, product of numerical aperture and corresponding fieldsize) for the effectively used field are presented in the following. Asexplained above, radiation enters the four-mirror-design at a mirrorgroup entry MGI geometrically close to a pupil surface (front pupilsurface FPS), and the mirror group exit MGO is also geometrically closeto a pupil surface (rear pupil surface RPS) indicating that the mirrorgroup performs a pupil imaging within the optical system. Further, eachof the mirror surfaces is positioned optically close to a field surface(intermediate image) in a sense that the mirror is optically closer to afield surface than to a pupil surface of the object system.Specifically, the absolute value of the chief ray height may be morethan twice the absolute value of the marginal ray height at the mirrorsurfaces. In order to avoid vignetting of the beam in the region of thepupil surface, the beam must pass the geometrically closest edge of themirrors forming the mirror group entry or mirror group exit. Regardingthe footprints of the beams on the mirrors care must be taken that theentire footprint falls on a reflective area of the mirror instead ofpassing the edge of a mirror, which would cause vignetting. A furtherpractical requirement is to obtain a sufficiently large object field asclose as possible to the optical axis in order to minimize the objectfield diameter for which the projection objective must be sufficientlycorrected for aberrations. Under these conditions, it has been founduseful to design the optical systems such that the size of the pupil(i.e. the beam diameter of the beam at a pupil surface) is as small aspossible at a pupil plane geometrically close to the mirror group entry(front pupil surface) and mirror group exit (rear pupil surface). Asmall pupil at the mirror group entry allows to place a geometricallyclose field (on or near an adjacent mirror) after an odd number ofreflections as close as possible to the optical axis without hitting themirror edge. Likewise, a small pupil at the mirror group exit allows toplace a geometrically close field (on or near an adjacent mirror) afteran even number of reflections as close as possible to the optical axiswithout hitting the mirror edge. Further considering that the product ofparaxial chief ray angle CRA and the size of a pupil is a constant in anoptical imaging system (Lagrange invariant) a small pupil corresponds tolarge chief ray angles at that pupil surface. In this context it hasbeen found useful for catadioptric in-line, systems having mirror groupsof the type shown here that the maximum chief ray angle CRA_(max) shouldexceed a critical value, thereby allowing to form a small pupil and anoblique beam path in the vicinity of a mirror group entry and mirrorgroup exit which, in turn, allows to place a large off-axis object fieldclose to the optical axis even at high numerical apertures.

The maximum chief ray angle CRA_(max) at the front pupil surface FPSclose to the mirror group entry is about 40° in FIG. 1. Useful valuesfor CRA_(max) may be in the range between about 20° and about 50°. Atlower values, the pupil size increases such that it becomes moredifficult to avoid vignetting without increasing the object fielddiameter to be corrected. At values higher than the upper limit, mirrorsurfaces may have to extend too far away from the optical axis therebyenlarging the mirror group size in radial direction and making mirrormanufacturing and mounting more difficult.

This basic type of design provides useful degrees of freedom withrespect to the amount of Petzval sum correction provided by the mirrorgroup. In order to demonstrate this flexibility, design variants havebeen created where the inner mirrors M2 and M3 are placed closertogether or further apart from each other when compared to thearrangement of FIG. 1. In that case, it is desirable to adapt thecurvatures of the mirrors such that the focal points of the innerconcave mirrors remain essentially at the related pupil positions. Inthis case, the Petzval sum is changing with increasing or decreasingmirror surface curvature, whereas the telecentric beam at the secondintermediate image between M2 and M3 is maintained due to symmetryreasons. Likewise, the pupil imaging property allowing the mirror groupto image the front pupil surface (close to the mirror entry) into therear pupil surface (close to the mirror group exit) remains essentiallystable.

FIG. 3 shows an example of a projection objective 300 designed as avariant to the embodiment of FIG. 1 where a larger difference betweenthe surface curvatures of the external mirrors M1, M4 on the one handand the internal mirrors M2, M3 on the other end is obtained.Specifically, the curvature radius of the inner mirrors M2, M3 isdecreased and the mirrors are closer together than in the embodiment ofFIG. 1. Comparing the embodiments of FIGS. 1 and 3 it can be seen that avariation of Petzval sum correction provided by the mirror group can beobtained within a concept of mirror symmetric arrangement of all fourmirrors of the mirror group with respect to the mirror group plane. Themirror group in the embodiment of FIG. 3 has the strongest effect onimage curvature due to the larger optical power of the concave mirrors.

Mirror groups designed symmetrically to a symmetry plane may beadvantageous from a manufacturing point of view since the samemanufacturing and testing devices can be used to manufacture more thanone concave mirror of the projection objective. Also, mounting can befacilitated.

In some cases, a larger flexibility for aberration control can beobtained when the mirror group is designed non-symmetric with respect toa mirror group plane MGP perpendicular to the optical axis. In theprojection objective 400 of FIG. 4 the symmetry between the mirrors isbroken and the second and fourth mirror M2, M4 are combined to form amirror pair on a single common mirror substrate providing one mirrorsurface with a defined surface of curvature common to both mirrors M2,M4. The mirror substrate has a transmissive portion TP (formed by a holein the mirror substrate) arranged such that the optical axis is includedand dimensioned to form the mirror group entry MGI where the projectionbeam provided by the first lens group LG1 near the front pupil surfacecan pass through the transmissive portion without hitting the edge ofthe hole (i.e. without vignetting). Using at least one mirror pair ofthis type helps to reduce the number of optical elements to bemanufactured and facilitates mounting of the concave mirrors formed onthe common substrate. In the embodiments all concave mirrors aredesigned as purely conical mirrors, whereby manufacturing and testing isfacilitated. In general, aspheric surfaces allow better aberrationcontrol.

In the projection objective 500 of FIG. 5, a similar system is shownequivalent to that of FIG. 4 in respect to the fact that one singlemirror pair formed on a common substrate is provided. Here, the imageside mirrors M1 and M3 are formed on a common substrate having a centralhole or bore providing a transmissive portion between the concavemirrors forming the mirror group exit MGO where the projection beampasses through when exiting the mirror group MG.

In another embodiment (not shown in the figures) the mirror group isformed of two mirror pairs, where the mirrors M1, M3 are formed on onesubstrate, and the mirrors M2 and M4 are formed on another substrate,each substrate having a central hole including the optical axis.

Another class of symmetry is demonstrated using the projection objective600 of FIG. 6 as an example. Here, the vertex positions of theobject-side mirrors M2 and M4 on the one hand and of the image-sidemirrors M1 and M3 on the other hand are identical. Therefore, theobject-side mirrors having their, mirror surfaces facing to theimage-side have the same axial position, but differ in surfacecurvature. Likewise, the image-side mirrors having the mirror surfacesfacing to the object have the same axial position, but differ in surfacecurvature. Mounting of the mirrors may be facilitated this way.

In FIG. 7 another variant of a projection objective 700 is shown whereno symmetry exists between the mirrors M1 to M4 of the mirror group MG.Specifically, each of the concave mirrors has a different surfacecurvature and a different vertex position. As the surface curvaturesdetermining the Petzval sum provided by a concave mirror and their axialpositions can be selected independently between the concave mirrors, adesired contribution of the mirror group Petzval sum correction can beobtained as desired.

Further characteristic features of the basic design type include thefollowing. High flexibility for the amount of Petzval sum correction asexplained. Due to the presence of an intermediate image (fourthintermediate image IMI4) outside the mirror group at a distancetherefrom, an accessible field surface is provided, facilitating thecorrection of aberrations and allowing to introduce a field stop, ifdesired. Small maximum lens diameters can be used due to the strongPetzval correction provided by the mirror group, thereby keeping theentire system mass and the amount of optical material for the lensesmoderate. The mirror group forms an axially compact unit within theprojection objective, keeping the overall track length moderate.

The embodiments shown in FIGS. 1 to 7 represent one preferred type ofmember of a design family having a mirror group having four nestedmirrors providing four reflections between mirror group entry and mirrorgroup exit. Specific properties of the design type have been explainedwith respect to the path described by the chief ray CR through themirror group. A systematic approach to characterize and qualify theentire design family of this type will now be explained in connectionwith FIG. 8. In this figure, a mirror group consisting of four concavemirrors is represented schematically by bracket-shaped curved lineshaving concave sides facing each other. The object-side mirrors M2, M4geometrically closer to the object surface OS and having mirror surfacesfacing the image surface IS are represented by a curved line convex tothe object surface, whereas the opposite image-side mirrors M1, M3 arerepresented by a curved line convex to the image surface. A radiationbeam passing through the mirror group between mirror group entry andmirror group exit is characterized by the chief ray CR. As explainedexemplarily in connection with FIGS. 1 to 7 various ways to design andarrange the concave mirrors are possible. In a general case, all fourmirrors are separate mirrors providing the highest degree of freedom forthe design since vertex positions and curvatures can be setindependently. Higher symmetries, e.g. one or two mirror pairs, equalvertex positions and/or curvature radii etc are possible.

Where a reflection of the beam occurs on the optical axis OA in theschematic figure, this represents a reflection optically close to apupil surface (abbreviated by R_(P)). In contrast, where a reflectionoccurs at a radial distance from the optical axis, this represents areflection closer to a field surface (R_(F)). A position where the chiefray CR intersects a curved line representing mirrors corresponds to aregion where the radiation beam enters or exits the mirror group. Atransmission close to the optical axis will be near a pupil anddesignated T_(P), whereas a transmission near a field surface will bedesignated T_(F).

FIGS. 8( a) to (d) represent four branches of the design familydifferent with respect to the position and direction of entry of theradiation beam into the mirror group. The branch in (a) is characterizedby a telecentric entry of the beam at MGI (i.e. chief ray parallel tooptical axis) close to a field surface outside the optical axis (T_(F))in level 0. The members of the branch in the following level 1 aregenerated depending on the position of the second reflection at M2,which may occur on the optical axis (Hp) or close to a field surface onthe opposite side of the optical axis (R_(F)). In FIG. 8( a) bothoptions in the first level are shown below each other.

The members of the second level following the first level are derivedfrom those of the first level depending on the position of thereflection subsequent to the second reflection occurring on theimage-side third mirror M3. In each of the two sub-branches following amember of the first level the reflection on the third mirror maytheoretically occur either in the image-side pupil position (R_(P)), orin the field position (R_(F)) not yet used on the image-side mirror,i.e. a field position on the opposite side to the first reflection. Itappears that among the four members of the second level, three membersappear to be physically feasible, whereas the fourth member (uppermostmember of level 2) characterized by a reflection on a pupil surfaceimmediately downstream of a reflection on a pupil surface appears to benot feasible. In FIG. 8, options which appear not feasible for physicalreasons are crossed, out.

The development of the members of the third level follows the sameprinciple for identifying what type or types of reflections appearfeasible on the object-side fourth mirror M4. The reflection must occurat a position not yet used on the object-side mirrors. For physicalreasons, two of the remaining three options in level 3 appear notfeasible, leaving only one option where a reflection on the fourthmirror M4 occurs near a field position (R_(F)) on the same side of theoptical axis as the third reflection upstream of the fourth reflection(R_(F)). In the fourth level, the system is completed when the beamreflected on the fourth mirror M4 exits the mirror group at the mirrorgroup exit MGO positioned around the optical axis, indicated by atransmission occurring near a pupil surface (T_(F)).

In summary, FIG. 8( a) shows that only one physically meaningful option(type A) remains if a telecentric beam enters a mirror group outside theoptical axis close to a field position (T_(F)). In this embodiment, thereflections and transmissions occurring on the object-side of the mirrorgroup may be characterized by the sequence: T_(F)-R_(P)-R_(F), whereasthe reflections and transmissions on the image-side of the mirror groupare characterized by: R_(F)-R_(F)-T_(P).

This notation illustrates that a radiation beam passing through themirror group is represented by three different positions of “footprints”of the beam in the region of the object-side mirrors and by threedifferent footprints in the region of the image-side mirrors. Using theobject-side mirrors M2, M4 as an example, one footprint occurs uponentry of the light beam into the mirror group (here optically remotefrom the optical axis (T_(F))), and two at subsequent reflections onobject-side mirrors (here R_(P) and, later, R_(F)).

In order to obtain an optical system free of vignetting, thesefootprints are not allowed to overlap. Instead, there must be a minimumdistance between the footprints. This is made possible by designing thesystem such that for each set of mirrors (mirrors M2 and M4 on theobject-side and mirrors M1 and M3 on the image-side) the chief rayheights for two reflections and one transmission are substantiallydifferent from each other. Therefore, only one footprint can include theoptical axis. This footprint (in transmission or in reflection) may beclose to a pupil surface. The remainder of the footprints (in reflectionor transmission) may not include the optical axis, indicating thetendency that these reflections or transmissions will be closer to afield surface, which may be an intermediate image.

In general, the positions of the footprints on the object side of themirror group in terms of chief ray heights may be characterized asfollows. The mirror group entry may be positioned in a region whereradiation exiting the front lens group has an entry chief ray height.The second reflecting area may be positioned in a region where radiationimpinging on the second mirror has a second chief ray height deviatingfrom the entry chief ray height in a first direction. The fourthreflecting area may positioned in a region where radiation impinging onthe fourth mirror has a fourth chief ray height deviating from the entrychief ray height in a second direction opposite to the first direction.

Embodiments of type A require an essentially telecentric input ofradiation, which can be provided by an imaging subsystem serving as arelay system arranged between object surface and mirror group entry.Considerable axial installation space may be required for that type ofrelay system.

Having explained the principle underlying the development of members ofthis design family depending on position and angle of the chief ray atthe mirror group entry, the other branches of the family are developedin a similar fashion. The branch illustrated in FIG. 8( b) ischaracterized by a chief ray entering the mirror group outside theoptical axis near a field surface (T_(F)) inclined to the optical axissuch that a first reflection at the first mirror M1 will occur near apupil surface (R_(P)). Two types of basic designs (type B and type C)may be derived for this type.

Embodiments of type B and C are characterized by a convergent beam onthe entry side of the mirror group and a divergent beam on the exit sideof the mirror group. Where an in-line system is required, relativelylarge lenses would be needed close to the mirrors, which may not bedesirable if a compact projection objective is wanted.

The branch of the design family depicted in FIG. 8( c) is characterizedby an off-axis mirror group entry and a convergent beam with atransmission near a field plane (T_(F)) followed by subsequentreflection on the first mirror M1 near a field surface on the oppositeside of the optical axis (R_(F)). Only one embodiment with fourreflections (type D) appears theoretically possible. Since an off-axisbeam strongly converging is required on the entry side, this variantappears less attractive for compact in-line catadioptric systems.

The fourth branch in FIG. 8( d) is characterized by a mirror group entryclose to the optical axis, preferably including that optical axis, suchthat a first transmission occurs near a pupil surface (T_(P)) and asubsequent first reflection close to a field surface (R_(F)). A totalnumber of four different types (E, F, G, H) can be obtained based onthis type of radiation entry. Briefly, type E is characterized by thefact that the mirror group exit MGO is positioned remote from theoptical axis such that a transmission near a field surface (T_(F))occurs at the mirror group exit. Both types F and H, in contrast, arecharacterized by the fact that a mirror group exit close to the opticalaxis, preferable including that optical axis, is present to allow thatthe radiation beam exists the mirror group in the vicinity of a pupilsurface (T_(P)). Whereas type G appears less attractive due to thestrongly diverging beam at the mirror group exit, types E, F and Happear to be attractive when designing catadioptric in-line projectionobjectives capable of transporting a large entendu of the effectiveobject field without vignetting at very high numerical apertures,particularly at numerical apertures allowing immersion lithography withimage-side numerical apertures NA>1. Preferred embodiments will beexplained below.

Other branches (not shown) are related to the branches shown in FIG. 8by mirror symmetry with respect to an axial plane perpendicular to themeridional plane (paper plane).

Some principles may be derived from the above considerations. Basically,it appears desirable that small footprints are obtained on the mirrorsurfaces. This appears advantageous with regard to the size of themirrors as well as to the size of the effective object field which canbe imaged through such system. Further, the footprints near pupilsurfaces in the region of the mirror group should be small to avoidvignetting of the beam at a mirror edge. Further, considering that theproduct of paraxial chief ray angle CRA and the size of a pupil is aconstant in an optical imaging system (Lagrange invariant), a smallpupil corresponds to large chief ray angles at that pupil surface.

Further, the systematic derivation of desirable variants of the designfamily allows to indicate useful features with respect to the paraxialconstruction (refractive/reflective powers and distances of the opticalelements) as well as with respect to the aspheric shape of the mirrorsurfaces. This will be explained in connection with FIG. 9 showing amirror group MG and a radiation beam entering and exiting a mirror groupat mirror group entry and mirror group exit, respectively, bothpositioned near a pupil surface of the system. A beam with a smallaperture is shown in FIG. 9( a), whereas a larger aperture beam is shownin FIG. 9( b) for the same mirror group.

In the notation explained above, the object-side mirrors M2, M4 arecharacterized by the sequence T_(P)-R_(F)-R_(F), whereas the image-sidemirror M1, M3 are characterized by the sequence R_(F)-R_(F)-T_(P). Twointermediate images are formed within the mirror group.

The object-side mirrors M2, M4 and the image-side mirrors M1, M3 eachhave the same vertex position, where an axial distance d exists betweenthe object-side and image-side vertex position. The radiation beamtransits the object-side mirrors in the vertex region around the opticalaxis and is collimated by first mirror M1 consistent with first mirrorM1 being a paraboloid mirror having a curvature radius r_(M1)=2d at thevertex. Second mirror M2 is designed to reflect the chief ray runningparallel to the optical axis such that the beam transits the center ofthe mirror group, indicating that second mirror M2 is a paraboloidhaving curvature radius r_(M2)=d. Likewise, r_(M3)=d and r_(M4)=2d forthe paraboloid mirrors M3 and M4. Having an object situated a suitabledistance upstream of the mirror group, the beam path shown in FIG. 9 canbe obtained. A mirror group of this type can be supplemented byadditional optical elements to form a catadioptric projection objectiveby providing a front lens group with positive refractive power upstreamof the mirror group entry MGI and a focusing group with positiverefractive power downstream of the mirror group exit MOO.

The following examples of embodiments are based on the design principleslaid out above. All embodiments have the same light conductance value(etendu) at a constant object field radius and image-side numericalaperture NA=1.20 suitable for immersion lithography. With regard tovignetting, the designs are optimized for an effective object fieldhaving rectangular shape and dimensions 26 mm×5.5 mm. The reductionratio is 4:1 (magnification |β|=0.25). One example (based on type H inFIG. 8) has already been discussed in detail in connection with FIGS. 1to 3, where variants are shown in FIGS. 4 to 7.

FIG. 10 shows a projection objective 1000 based on type F of FIG. 8,where the surfaces of curvature of first mirror M1 and fourth mirror M4have large radius indicating a small mirror sag. Here, the beamconverged by front lens group FLG enters the mirror group in thevicinity of a system pupil FPS and forms a first intermediate image IMI1on and/or near the first mirror M1. The beam reflected from the firstmirror to the second mirror M2 positioned on the same side at theoptical axis is essentially collimated by the second mirror M2 havingstrong surface curvature and transits a second pupil upon crossing theoptical axis. After reflection on strongly curved third mirror M3 andweakly curved fourth mirror M4 positioned on the same side of theoptical axis, the beam forms a second intermediate image IMI2 and exitsthe mirror group at mirror group exit MGO geometrically close to therear pupil surface RPS. All four concave mirror have an aspheric mirrorsurface.

Second lens group LG2 forms a third intermediate image IMI3 in theregion of the constriction CON before the beam is converged by the thirdlens group LG3 to form a high aperture beam converging on the imagesurface IS.

FIG. 11 shows a variant 1100 of the system of FIG. 10, where the mirrorsof the mirror group MG are disposed mirror symmetrically around themirror group plane MGP halfway between the most object-side and the mostimage-side mirror vertices. In this approach, the inner mirrors (closerto the mirror group plane) can be modified to further reduce the surfacecurvature. A limiting case is shown as objective 1200 in FIG. 12, wherethe first mirror M1 and the fourth mirror M4 are degenerated to formplanar mirrors aligned perpendicular to the optical axis. As planarmirrors are optically neutral with respect to introduction or removal ofoptical aberrations, planar mirrors are usually only used as foldingmirrors to deflect radiation and, thereby, to fold the optical axis ofoptical systems. Therefore, planar mirrors can be removed from the beampath without influencing the optical performance. However, vignettingproblems may be introduced upon removing a planar mirror. The projectionobjective 1300 shown in FIG. 13 is basically derived from the embodiment1200 of FIG. 12 by removing the planar mirrors M1, M4 and modifying thelens groups upstream and downstream of mirror group MG.

FIG. 14 shows the same projection objective 1300 where only theeffective parts of the concave mirrors M1 and M2 are shown to facilitateunderstanding construction of this projection objective. Thespecification is given in tables 14, 14A. The projection objective has afirst lens group LG1 acting as an imaging subsystem to form a firstintermediate image IMI1. A second, catoptric subsystem is formed by amirror group MG and designed to form a second intermediate image IMI2from the first intermediate. A second lens group LG2 is designed as animaging subsystem to form a third intermediate image IMI3 from thesecond intermediate image in a constriction region CON of the rear lensgroup RLG following the mirror group. A third lens group LG3 withpositive refractive power serves to image the intermediate image on areduced scale onto the image surface IS. The mirror MG consists of afirst concave mirror M1 having a mirror surface facing the object side,and a second concave mirror M2 having a mirror surface facing the imageside. A mirror group entry MGI is defined in the region where thecurvature surface defined by the second mirror intersects the opticalaxis OA, whereas a mirror group exit MGO is defined where the surface ofcurvature of the first mirror M1 intersects the optical axis. Bothintermediate images (at least the paraxial part thereof) are positionedwithin an intermirror space defined by the concave mirrors. Both mirrorsare arranged near an intermediate image, i.e. optically remote from apupil surface. The mirror group entry is positioned geometrically in anintermediate region between a front pupil surface downstream of the exitlens of the first lens group LG1 and the first intermediate image.Likewise, the mirror group exit is positioned intermediately between thesecond intermediate image and the subsequent pupil surface RPS positionin the entry section of the second lens group LG2.

It is a characterizing feature of this type of two-mirror in-lineprojection objective that the angle included between the chief ray CRand the optical axis (chief ray angle CRA) may be as high as 70° or 80°or more indicated by the fact that the radiation beam crosses theoptical axis between the first and second mirror almost at right angles.This corresponds to a small beam cross section at this pupil surface.High values for the chief ray angle are also obtained upstream anddownstream of the mirror group in the region of the first and secondintermediate images, respectively.

Although projection objectives including mirror groups according totypes B, C, D or G of FIG. 8 are theoretically possible, they appearless desirable for the following reasons. High values of the chief rayangle CRA next to an intermediate image are needed at the mirror groupentry and/or at the mirror group exit to obtain a strongly converging orstrongly diverging chief ray in order to obtain a small footprint in apupil surface. In order to obtain a strongly divergent or convergentchief ray at the mirror group entry outside the optical axis, the frontlens group arranged between optical surface and mirror group entry needsto have strong refractive power provided by large diameter lenses.Likewise, if a strongly divergent beam emerging from the mirror group atan off-axis mirror group exit is provided, large lenses providing strongrefractive power are needed to guide the beam towards the region of thepupil surface closest to the image surface, where an aperture stop maybe positioned. Also, a plurality of relatively large lenses appears tobe necessary to realize such systems. As a consequence, large lensdiameters and/or large system length may be necessary in such systems.

Systems of type A or E are basically equivalent to each other with adifference lying in the radiation propagation direction. In order toobtain a telecentric beam off-axis at the mirror, group entry (type A)an imaging subsystem serving as a relay system would be required betweenobject surface and mirror group, thereby increasing the system length inthis part. A beam having a large chief ray angle emanating from themirror group close to a pupil surface will normally require that thesystem part downstream of the mirror group is designed for creating anintermediate image, basically as described in connection with FIGS. 1 to7, for example.

Mirror groups of type E requiring large chief ray angles (or a smallpupil) at the mirror group entry and a telecentric off-axis beam at themirror group exit will now be explained in connection with FIGS. 15 to18. In each embodiment, a front lens group FLG serving as a relay systemto provide large chief ray angles and a small beam diameter at the frontpupil surface FPS close to the mirror group entry is provided. The axiallength may be relatively short. In each case, the mirror group MG isdesigned for providing four reflections between mirror group entry andmirror group exit, thereby forming two intermediate images IMI1, IMI2.The first, second and fourth reflection are optically remote from apupil surface, whereas the third reflection is near a pupil surface. Ineach case, the lenses downstream of the mirror group form an imagingsubsystem including a constriction region CON with a local minimum ofbeam diameter for imaging the second intermediate image IMI2 onto theimage surface IS. Relatively large lens diameters of lenses immediatelydownstream of the mirror group are one characterizing feature of theseembodiments required to “capture” the off-axis intermediate image and toconverge the beam towards the constriction region.

Embodiments of this type may be realized with two physically identicalaspheric mirrors having an essentially parabolic shape, where the amountof curvature radius at the vertex, |r|, equals twice the distance dbetween the vertices of the mirrors. FIGS. 15( a) and (b) and FIG. 16illustrate such embodiments. In FIG. 15( a) a low aperture beam isshown, whereas in (b) a similar beam is shown at higher numericalaperture to indicate the actual sizes of footprints expected in theregion of the front pupil surfaces FPS and in the reflecting areas onthe concave mirrors.

A projection objective 1600 with a telecentric beam at the secondintermediate image close to the mirror group ext MGI is shown in FIG. 16showing that large positive lenses with a diameter comparable to thediameter of the mirror group are required to focus the beam towards theimage surface.

In order to facilitate lens manufacturing and to reduce system mass,this problem can be alleviated by providing a fourth mirror M4 having alarger refractive power, whereby a chief ray significantly converging atthe third intermediate image is obtained (Projection objective 1700 inFIG. 17). Smaller lens diameters of the positive lenses immediatelydownstream of the mirror group exit are obtained. Note that thefootprints of the beam on the image-side mirrors M1 and M3 areoverlapping partially. This is made possible by constructing mirrors M1and M3 on a single mirror substrate having a common surface ofcurvature, whereby a multiply reflecting mirror is provided which isused twice at reflecting areas overlapping at least partially.

A further improvement with respect to smaller lens diameters immediatelydownstream of the mirror group is shown exemplarily for projectionobjective 1800 in FIG. 18, where the radius of curvature of the secondand fourth mirrors is further reduced when compared to the previousembodiments, thereby further increasing the chief ray angle to form aconvergent beam at the mirror group exit MGO. In this embodiment theimage-side mirrors M1, M3 are spherical with large curvature radius,whereas the object-side mirrors M2, M4 are aspherical. The specificationis given in tables 18, 18A

All embodiments presented so far have an axially compact mirror groupproviding strong Petzval overcorrection. In some embodiments, synergyeffects are obtained for manufacturing and testing since mirrorsidentical in construction are used, e.g. in mirror-symmetric mirrorgroups. Also, mirror pairs provided on a common substrate are employedin some embodiments, thereby facilitating manufacturing and mounting.Providing strongly curved mirrors for Petzval correction allows toreduce the number and maximum diameters of lenses, thereby reducingoverall dimension and mass consumption of the projection objectives. Insome embodiments, a real intermediate image is accessible in the systempart downstream of the mirror group, whereby correction of opticalperformance can be improved by applying a suitable field stop. In theembodiments without intermediate image downstream at the mirror group(e.g. FIGS. 17, 18) relatively flat concave mirrors can be used, whichfacilitates manufacturing of the mirrors.

All systems are designed for projecting a rectangular effective objectfield onto the image surface. Improvements with respect to opticaldesign can be obtained if an arcuate field (also denoted “ring field” or“annular field”) is used.

In the above mentioned embodiments having four mirrors, the mirrors arenested into each other such that the mirror group plane inside themirror group is transited five times, thereby allowing to constructaxially short mirror groups. In the following, catadioptric projectionobjectives suitable for immersion lithography are shown schematically,where other arrangements of mirror groups having four consecutivemirrors along a propagation of the projection beam are shown.

The projection objective 1900 of FIG. 19 has a refractive front lensgroup FLG following the object surface for forming a first intermediateimage from the object field. A second, purely reflective imagingsubsystem formed by a first mirror M1 and a second mirror M2 havingfacing concave mirror surfaces is designed to form a second intermediateimage IMI2. A third catoptric (purely reflective) imaging subsystemformed by a third mirror M3 and a fourth mirror M4 having facing concavemirror surfaces is designed to form a third intermediate image IMI3,which is then focused onto the image surface IS by a refractive rearlens group RLG. The mirror group entry MGI of the mirror group MG isformed next to second mirror M2 optically close to the firstintermediate image, whereas the mirror group exit MGO is formed on thesame side of the optical axis OA next to third mirror M3 optically nearthe third intermediate image IMI3. The front lens group FLG allows toposition the first intermediate image IMI1 at an optimum position nextto the second mirror and to shape the beam in the region of the mirrorgroup entry such that beam vignetting at the second mirror M2 isavoided. Using an annular field instead of a rectangular field cancontribute to avoiding vignetting. As all concave mirrors M1 to M4 arepositioned optically closer to field surfaces (at intermediate images)than to pupil surfaces (at intersections between the chief ray CR andthe optical axis OA), mirrors with compact size can be used, furthercontributing to define an optical path where a larger aperture beam froma large object field can be guided through the mirror group withoutvignetting. The off-axis object field can, therefore, be positionedrelatively close to the optical axis, whereby the size of the objectfield which must be corrected sufficiently, can be minimized.

As demonstrated by the projection objective 2000 of FIG. 20, the basicdesign can be modified to include one or more lenses optically withinthe mirror group such that a catadioptric imaging subsystem imaging thefirst intermediate image into the third intermediate image is obtained.For example, a lens or lens group L1 may be positioned geometricallybetween the first and second mirror in order to influence the correctionstatus of second intermediate image IMI2. One or more lenses used twiceor three times in transmission between the first and second intermediateimage may be used. Alternatively, or in addition, a lens or lens groupL2 may be arranged geometrically between the fourth and third mirror toinfluence the imaging from second intermediate image IMI2 to thirdintermediate image IMI3.

Preferably, a positive lens group L12 may be arranged optically betweenthe second and third mirrors M2, M3 close to the second intermediateimage IMI2 in order to optimize the transition between the second andthird imaging subsystems.

If a compact size particularly on the object side is desired, a compactfront lens group FLG designed as a Fourier lens group for creating afront pupil surface FPS near the exit of the front lens group may beprovided, as shown schematically for the projection objective 2100 ofFIG. 21. This type of front lens group may be combined with each of thevariants of mirror groups shown in FIG. 19, 20 or 21.

The projection objective 2200 of FIG. 22 is designed to image the objectfield from the object surface OS onto the image surface IS whilecreating exactly one intermediate image IMI1 inside the mirror group MGconsisting of four mirrors M1 to M4. In this embodiment, a firstcatadioptric imaging subsystem is formed by the refractive front lensgroup FLG having positive refractive power, a convex mirror M1 providingnegative refractive power, and the subsequent concave mirror M2 havingpositive refractive power. The first intermediate image IMI2 is imagedby the catadioptric second subsystem formed by concave mirror M3 havingpositive refractive power, convex mirror M4 providing negativerefractive power, and a refractive rear lens group RLG having positiverefractive power. The inner mirrors M1, M4 of the mirror group MG may beformed on one common mirror substrate, e.g. a substrate shaped as abiconvex half-lens having convex surfaces coated with a reflectioncoating.

In the embodiments of FIGS. 19 to 21, the consecutive concave mirrorsM1, M2 of the second subsystem, and M3, M4 of the third subsystem arepositioned on opposite sides of the optical axis, thereby creating aZ-shaped beam path between subsequent intermediate images, where theprojection beam crosses the optical axis at relatively high angles (e.g.between 30° and 60°). In contrast, the mirrors M1 to M4 of the mirrorgroup of projection objective 2200, are arranged on one side of theoptical axis only. This arrangement facilitates providing projectionobjectives which can be used with two separate optical channels oneither side of the optical axis, as explained below in connection withFIGS. 23 and 24.

The double-channel projection objective 2300 in FIG. 23 combines twooptically identical optical channels of the type shown in FIG. 22. Thefirst optical channel includes, in that sequence, the positive frontlens group FLG, convex mirror M1-1, concave mirror M2-1, concave mirrorM3-1, convex mirror M4-1, and the refractive positive rear lens groupRLG. The optical elements for the second channel are arrangedmirror-symmetrically to these optical elements with respect to a mirrorplane including the optical axis and aligned perpendicular to themeridional plane shown in the drawings. Therefore, the second channelincludes, in that sequence, the positive front lens group FLG (used forboth optical channels), a convex first mirror M1-2, a concave secondmirror M2-2, a concave third mirror M3-2, a convex fourth mirror M4-2,and the positive refractive rear lens group RLG used in both opticalchannels. The inner mirrors M1-1, M1-2, M4-1 and M4-2 may be provided onone common mirror substrate shaped like a biconvex positive lens. Also,the object side concave mirrors M2-1 and M2-2 may be provided on onecommon mirror substrate having a transmissive portion provided by a holeor the like to form the mirror group entry MGI around the optical axisOA. Likewise, the image-side concave mirrors M3-1 and M3-2 may also beformed on a common mirror substrate having a central opening forming themirror group exit MGO.

As demonstrated schematically in FIG. 24, a double-channel catadioptricprojection objective of the basic type shown in FIG. 24 allows to imagetwo identical off-axis object fields OF1, OF2 disposed on opposite sidesof the optical axis OA when viewed in the meridional sectionsimultaneously onto two identical image fields. The effectively usedobject field may be rectangular, or annular, as shown in FIG. 24( a) or(b), respectively.

Both optical channels may be used simultaneously. One optical channelmay, for example, be used for imaging a pattern on a reticle onto aphotosensitive substrate arranged in the image surface. The otheroptical channel may be used for measuring purposes, thereby forming apart of an optical measuring system for focus detection or for acquiringother measuring data useful for operating a microlithographic projectionsystem.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allchanges and modifications as fall within the spirit and scope of theinvention, as defined by the appended claims, and equivalents thereof.

The contents of all the claims is made part of this description byreference.

TABLE 1 (k25) SURFACE RADIUS THICKNESS MATERIAL INDEX SEMIDIAM. 00.000000 71.809371 75.0 1 −327.637978 32.132494 SILUV 1.560491 93.5 2−169.135759 0.996209 97.3 3 193.567383 45.881322 SILUV 1.560491 101.4 4−950.104533 40.297223 99.3 5 332.998259 22.215367 SILUV 1.560491 79.9 6−250798.749340 1.558868 76.0 7 147.021716 31.349085 SILUV 1.560491 66.58 −269.376838 3.738879 60.2 9 −574.049170 12.787736 SILUV 1.560491 55.410 −358.379807 310.085925 50.1 11 −205.152630 −261.958073 REFL 194.6 12202.931962 243.803700 REFL 133.4 13 −202.931962 −261.958073 REFL 137.114 205.152630 313.323179 REFL 189.7 15 333.723117 21.965974 SILUV1.560491 43.8 16 −148.735190 1.027909 48.7 17 297.237468 17.668324 SILUV1.560491 57.4 18 −22655.613711 0.996972 60.6 19 266.059259 9.996029SILUV 1.560491 64.6 20 167.429960 17.644922 67.2 21 316.769307 45.337134SILUV 1.560491 75.9 22 −141.417016 1.734111 79.0 23 180.402896 43.553038SILUV 1.560491 81.2 24 −229.551725 0.994573 79.7 25 106.098586 21.908375SILUV 1.560491 61.3 26 207.804050 37.790308 56.4 27 149.875422 32.465788SILUV 1.560491 51.0 28 96.401231 25.902263 50.9 29 −805.354747 12.377317SILUV 1.560491 55.6 30 −291.950807 1.619530 59.6 31 5643.34620825.483686 SILUV 1.560491 61.9 32 359.386131 17.719936 73.0 33−484.371794 43.227135 SILUV 1.560491 76.5 34 −111.090269 0.983849 83.135 −282.651715 16.686529 SILUV 1.560491 88.7 36 −193.331408 14.59577993.1 37 1289.507216 36.402691 SILUV 1.560491 104.6 38 −290.0565595.818675 106.2 39 294.615329 37.984725 SILUV 1.560491 106.1 40−883.131044 4.637061 104.5 41 0.000000 3.884710 101.3 42 304.45254032.963227 SILUV 1.560491 100.0 43 −1175.091696 0.988159 98.0 44205.061398 28.472058 SILUV 1.560491 89.8 45 −1772.564195 0.942761 86.146 88.191089 36.108906 SILUV 1.560491 69.2 47 217.978071 0.871387 60.348 51.039320 40.196538 SILUV 1.560491 43.4 49 0.000000 3.000000 WATER1.430000 23.6 50 0.000000 0.000000 18.8

TABLE 1A Aspheric Constants SURFACE 2 5 8 11 13 K  0  0  0 −0.224372−1.25495 C1 −5.372092E−08 −4.168360E−08  2.254214E−07 −2.785907E−10−1.081432E−08 C2  3.653441E−12  1.467973E−11  1.385239E−11  4.167344E−14−1.066392E−13 C3 −2.693024E−16 −4.220656E−16 −1.339238E−15 −2.118652E−18 1.086705E−17 C4  1.948646E−20 −1.238484E−19  9.377023E−20  4.815732E−23−5.881454E−22 C5 −8.492783E−25  5.618258E−24 −2.932861E−23 −4.290162E−28 1.272860E−26 SURFACE 15 30 35 45 47 K  0  0  0  0  0 C1 −3.171151E−07 3.087830E−07 −1.035707E−07  9.881636E−08  3.873571E−08 C2  1.900253E−11 1.777935E−11  2.333117E−12 −5.495318E−12  1.564412E−11 C3 −5.773786E−15−2.401684E−15 −9.110880E−18  5.554735E−16 −4.363822E−15 C4  1.930642E−19 1.697076E−19  5.617531E−21 −3.421920E−20  7.459036E−19 C5  6.320682E−23−1.527037E−22  9.796509E−25  1.357995E−24 −6.088648E−23

TABLE 14 (k34) SURFACE RADIUS THICKNESS MATERIAL INDEX SEMIDIAM. 00.000000 71.724752 75.0 1 −4121.411157 46.865076 SILUV 1.560383 97.2 2−149.962898 3.423696 99.7 3 −367.252336 29.694352 SILUV 1.560383 97.9 4−182.570909 48.276329 98.7 5 959.073897 16.529732 SILUV 1.560383 74.8 6−488.993602 1.002605 73.1 7 128.750652 36.221741 SILUV 1.560383 65.2 8−280.818471 219.182223 61.1 9 −113.350282 −168.653496 REFL 115.9 10113.350282 211.312559 REFL 119.9 11 1682.892626 38.461666 SILUV 1.56038352.7 12 −115.634084 1.199577 61.0 13 297.973893 28.937392 SILUV 1.56038373.7 14 −377.238203 1.001567 75.8 15 139.258715 30.708005 SILUV 1.56038383.0 16 387.895733 1.004927 81.7 17 110.677937 31.392930 SILUV 1.56038380.4 18 197.438459 20.626685 76.0 19 98.062272 27.890402 SILUV 1.56038365.5 20 195.590036 18.028744 59.3 21 −335.659469 9.999763 SILUV 1.56038356.7 22 4264.334463 40.862649 52.4 23 −276.324463 29.760282 SILUV1.560383 66.9 24 −112.588669 0.999906 72.0 25 −391.255584 31.195933SILUV 1.560383 76.3 26 −129.691815 37.478507 78.8 27 −257.7035959.999895 SILUV 1.560383 74.6 28 −938.678312 8.021413 75.7 29 −559.5084279.999819 SILUV 1.560383 76.0 30 166.815587 32.285194 79.4 31 −414.35092532.423065 SILUV 1.560383 81.9 32 −147.795503 0.999792 87.4 33−644.590081 21.574078 SILUV 1.560383 94.6 34 −296.542816 0.999864 97.735 3311.074462 21.600111 SILUV 1.560383 100.9 36 −465.020970 1.496340103.1 37 255.249942 32.190038 SILUV 1.560383 108.8 38 1404.84012837.695060 107.9 39 0.000000 −15.202638 107.4 40 277.432344 31.650721SILUV 1.560383 108.0 41 2570.652765 4.962705 106.8 42 258.80994732.551744 SILUV 1.560383 104.2 43 3849.847647 0.999900 101.9 44182.274330 26.041991 SILUV 1.560383 94.3 45 499.477958 0.999732 89.6 46128.727133 40.872544 SILUV 1.560383 82.0 47 −1207.592715 0.997468 75.448 54.278105 48.712729 SILUV 1.560383 47.8 49 0.000000 3.000000 WATER1.430000 23.4 50 0.000000 0.000000 18.8

TABLE 14A Aspheric Constants SURFACE 2 5 8 10 11 K  0  0  0 −0.262667  0C1 −1.436094E−08 −1.146558E−07 −2.513372E−08 4.202263E−09 −1.544236E−07C2  1.736738E−12 −3.677693E−12  9.109044E−12  1.189315E−13  1.308220E−11C3  1.652186E−16  3.300817E−16  4.411748E−16  6.423802E−18 −1.220101E−15SURFACE 28 35 45 47 K  0  0  0  0 C1  1.560789E−07 −2.585328E−08 7.159379E−08  9.964302E−08 C2 −2.463134E−11 −4.603895E−13 −1.225190E−12−2.343372E−12 C3  1.301161E−15 −3.126818E−17  4.705364E−17  1.692824E−16

TABLE 18 (k39) SURFACE RADIUS THICKNESS MATERIAL INDEX SEMIDIAM. 00.000000 43.647197 75.0 1 480.810746 22.450775 SILUV 1.560383 91.5 2−2601.917214 0.999999 92.9 3 244.716473 42.646541 SILUV 1.560383 96.0 4−475.459049 5.206433 96.4 5 −65081.942196 30.369493 SILUV 1.560383 95.36 −282.091757 4.319503 94.7 7 4054.660864 21.027317 SILUV 1.560383 88.48 −425.538135 1.058617 86.1 9 141.770075 43.413138 SILUV 1.560383 74.010 −504.879053 228.016645 63.4 11 −16139.952712 −191.788330 REFL 222.912 279.609418 191.788330 REFL 259.2 13 −16139.952712 −191.788330 REFL140.0 14 279.609418 222.223946 REFL 216.7 15 595.895710 11.551775 SILUV1.560383 164.0 16 493.518105 0.999953 160.3 17 167.407331 52.914514SILUV 1.560383 143.3 18 257.576900 0.999867 136.3 19 169.77134448.837857 SILUV 1.560383 125.3 20 383.581700 57.475068 116.2 211366.696104 10.061298 SILUV 1.560383 76.7 22 104.842690 46.789200 65.323 −218.633249 10.006978 SILUV 1.560383 62.9 24 123.981552 28.30168863.1 25 −420.007166 23.760089 SILUV 1.560383 64.8 26 −136.76020420.382264 69.2 27 −87.639997 11.719453 SILUV 1.560383 70.2 2811545.411095 16.873327 96.9 29 −421.532046 61.462388 SILUV 1.560383101.8 30 −131.126498 1.164116 111.0 31 −4703.328932 42.525814 SILUV1.560383 136.9 32 −322.106158 18.815740 140.3 33 566.126522 59.056558SILUV 1.560383 151.9 34 −681.660146 0.999644 151.9 35 419.17264038.681402 SILUV 1.560383 146.8 36 −89485.147274 1.762260 144.9 370.000000 −0.762294 144.4 38 339.457499 31.487971 SILUV 1.560383 138.4 39918.153311 0.999905 135.1 40 263.015881 18.899856 SILUV 1.560383 127.341 351.235101 0.999907 122.4 42 180.421341 26.449443 SILUV 1.560383116.3 43 226.435883 0.999902 110.0 44 155.314905 73.471946 SILUV1.560383 104.0 45 2619.914961 0.999776 80.8 46 62.797187 54.721118 SILUV1.560383 53.4 47 0.000000 3.000000 WATER 1.430000 23.3 48 0.0000000.000000 18.8

TABLE 18A Aspheric Constants SURFACE 3 5 8 10 12 K  0  0  0  0 −0.218707C1 −5.326670E−08  3.048880E−08  4.205559E−08  3.924322E−07  0.000000E+00C2  5.331856E−12 −1.401988E−11 −7.663654E−12 −5.732989E−11  0.000000E+00C3 −4.494010E−16  9.117764E−16  6.020378E−16  3.684328E−15  0.000000E+00SURFACE 16 25 41 45 K  0  0  0  0 C1 −5.147358E−09 −1.191436E−07 1.653223E−08  7.662196E−08 C2 −4.097558E−14  5.549394E−13  6.240040E−14−2.469195E−12 C3  1.839836E−18  1.767377E−16 −3.065806E−19  1.239093E−16

1. A catadioptric projection objective for imaging an off-axis objectfield arranged in an object surface of the projection objective onto anoff-axis image field arranged in an image surface of the projectionobjective while creating at least one intermediate image comprising inthat order along an optical axis: a front lens group having positiverefractive power for converging radiation coming from the object fieldtowards a mirror group entry of a mirror group; the mirror group havingthe object side mirror group entry, an image side mirror group exit, anda mirror group plane defined transversely to the optical axis andarranged geometrically between the mirror group entry and the mirrorgroup exit; and a rear lens group with positive refractive power forfocusing radiation emerging from the mirror group exit onto the imagesurface; the mirror group having: a first mirror for receiving radiationfrom the mirror group entry on a first reflecting area; a second mirrorfor receiving radiation reflected from the first mirror on a secondreflecting area; a third mirror for receiving radiation reflected fromthe second mirror on a third reflecting area; and a fourth mirror forreceiving radiation reflected from the third mirror on a fourthreflecting area and for reflecting the radiation to the mirror groupexit; at least two of the mirrors being concave mirrors having a concavemirror surface having a surface of curvature rotationally symmetric tothe optical axis; wherein: the mirrors of the mirror group are arrangedsuch that at least one intermediate image is positioned inside themirror group between mirror group entry and mirror group exit, and thatradiation coming from the mirror group entry passes at least four timesthrough the mirror group plane and is reflected at least twice at aconcave mirror surface of the mirror group prior to exiting the mirrorgroup at the mirror group exit; the mirror group entry is positioned ina region where radiation exiting the front lens group has an entry chiefray height; the second reflecting area is positioned in a region whereradiation impinging on the second mirror has a second chief ray heightdeviating from the entry chief ray height in a first direction; and thefourth reflecting area is positioned in a region where radiationimpinging on the fourth mirror has a fourth chief ray height deviatingfrom the entry chief ray height in a second direction opposite to thefirst direction.
 2. Projection objective according to claim 1, whereinthe mirror group is designed such that exactly four reflections occurwithin the mirror group.
 3. Projection objective according to claim 1,wherein the first, second, third and fourth mirror is a concave mirror.4. Projection objective according to claim 1, wherein the mirror groupis a purely reflective (catoptric) mirror group.
 5. Projection objectiveaccording to claim 1, wherein a length ratio LR between an axial mirrorgroup length MGL and a total track length TT of the projection objectiveis less than 50%, where the mirror group length is the axial distancebetween a mirror vertex closest to the object surface and a mirrorvertex closest to the, image surface and the total track length is theaxial distance between object surface and the image surface. 6.Projection objective according to claim 5, wherein the conditionLR=MGL/TT<30% is fulfilled.
 7. Projection objective according to claim1, wherein the mirror group entry includes the optical axis andpositions of the chief ray in the second and fourth reflecting areas arepositioned on opposite sides of the optical axis.
 8. Projectionobjective according to claim 1, wherein the mirror group entry ispositioned geometrically close to a front pupil surface of theprojection objective such that a radiation beam emanating from theobject field includes the optical axis in the region of the mirror groupentry.
 9. Projection objective according to claim 8, wherein the frontlens group is designed as a Fourier lens group for performing one singleFourier transformation or an odd number of consecutive Fouriertransformations between the object surface and the mirror group entry.10. Projection objective according to claim 9, wherein the front lensgroup is purely refractive and performs a single Fourier transformation.11. Projection objective according to claim 1, wherein the front lensgroup is axially compact having an axial length which is less than 40%of a total track length of the projection objective.
 12. Projectionobjective according to claim 1, wherein the mirror group exit isarranged geometrically close to a rear pupil surface optically conjugateto the front pupil surface.
 13. Projection objective according to claim1, wherein one of two and three intermediate images are formed withinthe mirror group.
 14. Projection objective according to claim 1, whereinthe mirror group entry is positioned geometrically close to a frontpupil surface of the projection objective such that a radiation beamemanating from the object field includes the optical axis in the regionof the mirror group entry, wherein the mirror group exit is arrangedgeometrically close to a rear pupil surface optically conjugate to thefront pupil surface, and wherein one of two and three intermediateimages are formed within the mirror group.
 15. Projection objectiveaccording to claim 1, wherein the mirror group exit is arrangedgeometrically close to a rear pupil surface, the rear lens groupincludes a Fourier lens group for forming an intermediate image in aconstriction region of the rear lens group, and a lens group downstreamof the intermediate image is designed as an imaging subsystem forimaging the intermediate image formed by the Fourier lens group onto theimage surface on a reduced scale.
 16. Projection objective according toclaim 1, wherein the mirror group exit is arranged outside the opticalaxis optically close to an intermediate image and wherein the rear lensgroup is designed as an imaging subsystem for imaging that intermediateimage onto the image surface on a reduced scale.
 17. Projectionobjective according to claim 16, wherein the rear lens group includes aconstriction region defining a local minimum of beam diameter and atleast two negative lenses are arranged in the constriction region. 18.Projection objective according to claim 1, wherein the mirror groupincludes at least one mirror pair consisting of two concave mirrorshaving mirror surfaces sharing a common surface of curvature provided ona common mirror substrate having a transmissive portion provided betweenthe concave mirrors of the mirror pair.
 19. Projection objectiveaccording to claim 18, wherein the mirror pair is arranged such that thetransmissive portion of the mirror pair includes the optical axis. 20.Projection objective according to claim 18, wherein the transmissiveportion is formed by a hole in a mirror substrate.
 21. Projectionobjective according to claim 1, wherein all reflecting areas on themirrors of the mirror group are positioned outside the optical axis. 22.Projection objective according to claim 1, wherein all reflecting areason the mirrors of the mirror group are positioned optically remote froma pupil surface.
 23. Projection objective according to claim 1, whereinan aperture stop is positioned upstream of a last intermediate imageclosest to the image surface.
 24. Projection objective according toclaim 1, wherein the catadioptric projection objective has an image sidenumerical aperture NA>0.8.
 25. Projection objective according to claim1, wherein the catadioptric projection objective is designed as animmersion objective adapted with reference to aberrations such that animage side working distance between a last optical element and the imageplane is filled up with an immersion medium with a refractive indexsubstantially greater than
 1. 26. Projection objective according toclaim 1, wherein the catadioptric projection objective has an image sidenumerical aperture NA>1.1 when used in connection with an immersionmedium.
 27. Projection objective according to claim 1, wherein thecatadioptric projection objective is configured for use with ultravioletlight falling within a wavelength range extending from about 120 nm toabout 260 nm.
 28. A projection-exposure system for use inmicrolithography having an illumination system and a catadioptricprojection objective, wherein the projection objective includes acatadioptric projection objective according to claim
 1. 29. A method forfabricating semiconductor devices or other types of microdevices,comprising: providing a mask having a prescribed pattern; illuminatingthe mask with ultraviolet light having a prescribed wavelength; andprojecting an image of the pattern onto a photosensitive substratearranged in the vicinity of the image plane of a projection objectiveusing a catadioptric projection objective according to claim
 1. 30. Acatadioptric projection objective for imaging an off-axis object fieldarranged in an object surface of the projection objective onto anoff-axis image field arranged in an image surface of the projectionobjective while creating at least one intermediate image comprising inthat order along an optical axis: a first imaging subsystem for imagingthe object field provided in the object surface into a firstintermediate image; a second imaging subsystem for imaging the firstintermediate image into a second intermediate image; a third imagingsubsystem for imaging the second intermediate image into a thirdintermediate image; a fourth imaging subsystem for imaging the thirdintermediate image onto the image plane; wherein the second subsystemincludes a mirror group having a first concave mirror having a firstcontinuous mirror surface and a second concave mirror having a secondcontinuous mirror surface facing the first mirror surface; and allconcave mirrors are arranged optically remote from a pupil surface. 31.Projection objective according to claim 30, wherein the second subsystemis a catoptric mirror group consisting of the first concave mirrorhaving a first continuous mirror surface facing the object surface andthe second concave mirror having a second continuous mirror surfacefacing the first mirror surface. 32-35. (canceled)
 36. A catadioptricprojection objective for imaging an off-axis object field arranged in anobject surface of the projection objective onto an off-axis image fieldarranged in an image surface of the projection objective while creatingone intermediate image comprising in that order along an optical axis: afirst catadioptric imaging subsystem for imaging the object fieldprovided in the object surface into the intermediate image; a secondcatadioptric imaging subsystem for imaging the intermediate image ontothe image surface; wherein the first catadioptric imaging subsystemincludes in that order a refractive front lens group having positiverefractive power, a convex mirror, and subsequent concave mirror, andthe catadioptric second subsystem includes in that order a concavemirror, a convex mirror, and a refractive rear lens group havingpositive refractive power.
 37. Projection objective according to claim36, wherein the convex mirror and the subsequent concave mirror of thefirst catadioptric imaging subsystem are arranged between a front pupilsurface and the intermediate image and the convex mirror and thesubsequent concave mirror of the second catadioptric imaging subsystemare arranged between the intermediate image and a rear pupil surface.38. A catadioptric projection objective for imaging an off-axis objectfield arranged in an object surface of the projection objective onto anoff-axis image field arranged in an image surface of the projectionobjective while creating at least one intermediate image comprising: anoptical axis; a first set of optical elements including at least oneconcave mirror for forming a first optical channel on a first side ofthe optical axis; and a second set of optical elements including atleast one concave mirror for forming a second optical channel on asecond side of the optical axis opposite to the first side. 39.Catadioptric projection objective according to claim 38, wherein theoptical elements of the first set and the optical elements of the secondset are arranged mirror-symmetrically to a symmetry plane perpendicularto a meridional plane of the projection objective and including theoptical axis.